1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material, and a recording method and a recording apparatus therefor, and more particularly to a reversible thermosensitive recording material which utilizes a coloring reaction of an electron donating coloring agent and an electron accepting coloring developer and in which a colored image is repeatedly formed and erased by appropriately heating and cooling the reversible thermosensitive recording material, and a recording method and a recording apparatus therefor.
2. Discussion of the Related Art
A variety of thermosensitive recording materials are well known in which a colored image can be formed by a coloring reaction when an electron donating coloring agent is brought into contact with an electron accepting coloring developer while heat or the like is applied thereto.
The thermosensitive recording materials have the following advantages over other conventional recording materials:
(1) color images can be rapidly recorded by a relatively simple apparatus without using such complicated steps as developing and fixing; PA0 (2) color images can be recorded without producing noise and environmental pollution; PA0 (3) various color images, e.g., red, blue, violet and black, can be easily obtained; PA0 (4) image density and background whiteness are high; and PA0 (5) the manufacturing cost is relatively low. PA0 (1) a thermosensitive recording material which includes gallic acid in combination with phloroglucinol serving as coloring developers (Japanese Laid-Open Patent Publication No. 60-193691); PA0 (2) a thermosensitive recording material which includes phenolphthalein or Thymolphthalein serving as a coloring developer (Japanese Laid-Open Patent Publication No. 61-237684); PA0 (3) a thermosensitive recording material which includes an uniform solid solution of a coloring agent, a coloring developer and a carboxylic acid ester (Japanese Laid-Open Patent Publications No. 62-138556, 62-138568 and 62-140881); PA0 (4) a thermosensitive recording material which includes an ascorbic acid derivative serving as a coloring developer (Japanese Laid-Open Patent Publication No. 63-173684); and PA0 (5) a thermosensitive recording material which includes bis (hydroxyphenyl) acetic acid or a higher aliphatic amine salt of gallic acid serving as a coloring developer (Japanese Laid-Open Patent Publication No. 2-188294).
Because of these advantages, the above-described thermosensitive recording materials can be widely used, not only as a recording material for price labels in stores, but also as recording materials for copiers, printers for computers, facsimiles, automatic vending machines of tickets, video printers and measuring instruments.
However, the coloring reaction of these thermosensitive recording materials is irreversible and accordingly these recording materials cannot be repeatedly used.
Several thermosensitive recording materials which can reversibly form and erase an image have been proposed. For example, they are as follows:
Some of the present inventors, jointly with others, have proposed a reversible thermosensitive coloring composition which includes a coloring agent such as a leuco dye and a coloring developer such as an organic phosphate compound, an aliphatic carboxylic acid compound or a phenolic compound each of which has a long-chain aliphatic hydrocarbon group, and a reversible thermosensitive recording material using the coloring composition (Japanese Laid-Open Patent Publication No. 5-124360). The reversible thermosensitive recording material can stably repeat image formation and image erasure by being appropriately heated and cooled, and the image-recorded state or the image-erased state can be stably maintained at room temperature. In addition, Japanese Laid-Open Patent Publication No. 6-210954 has disclosed a reversible thermosensitive recording material which includes a specified phenolic compound having a long-chain aliphatic hydrocarbon group serving as a coloring developer.
Thus reversible thermosensitive recording materials have been proposed; however, these recording materials do not have requisite properties of quick erasability and good preservability when they are used under various environmental conditions of temperature and humidity or when light is irradiated thereto. In attempting to improve the preservability, Japanese Laid-Open Patent Publications No. 7-164746 and 7-179043 have disclosed reversible thermosensitive recording materials which include a specified phenolic compound as a coloring developer. However, these recording materials cannot improve the preservability because an image recorded in the recording material cannot be clearly erased when the image is erased after light of 5000 lux is irradiated to the image for 100 hours. When these recording materials are in practical use in the market, a serious problem may occur in which information recorded in the recording materials is misunderstood.
In addition, these reversible thermosensitive recording materials have a drawback in that the image density deteriorates or the recording layer becomes deformed when images are repeatedly formed and erased in various environmental conditions of practical use of the recording materials (a problem hereinafter termed "deformation"). The reason for the deformation is considered to be that the structures of the recording layer and the protective layer of the recording material are gradually changed and deteriorated by the mechanical force and the heat of a thermal printhead which applies heat to the recording material to form or erase images.
In attempting to solve this problem, a reversible thermosensitive recording material has been disclosed which has a recording layer including particles whose average particle diameter is more than 1.1 times the thickness of the recording layer (Japanese Laid-Open Patent Publication No. 6-340171). In addition, a reversible thermosensitive recording material has been disclosed which has good ability to be used with thermal printheads owing to the formation of a protective layer thereon which has specified gloss and surface smoothness (Japanese Laid-Open Patent Publication No. 8-156410). However, these recording materials cannot yet entirely prevent the deterioration of the recording layer and the protective layer, which results in occurrence of deformation of the recorded image when the recording materials are repeatedly used. Therefore, these recording materials have a short life.
Further, Japanese Laid-Open Patent Publication No. 8-132738 has disclosed a reversible thermosensitive recording material which includes a compound selected from the group consisting of amide compounds, ester compounds, urea compounds and ketone compounds together with a dye having a lactone ring which serves as a coloring agent, an irreversible coloring developer having a phenolic hydroxy group and a resin having color erasability. This recording material is different from the reversible thermosensitive recording material which utilizes a coloring/decolorizing reaction of a coloring agent and a coloring developer and to which the present invention relates. In addition, this recording material has a drawback in that it takes a long time (about 30 seconds) to erase an image and therefore rapid image erasure is difficult when using the recording material.
Because of these reasons, a need exists for a reversible thermosensitive recording material which has such good image formation/erasure ability, good preservability, and good durability as to be able to be repeatedly used in market for a long time.